1. Field of the Invention
The present invention relates to a process for producing hexahydrothieno[3,4-]imidazole-2,4-diones. More particularly, the present invention concerns a process for producing hexahydrothieno[3,4-d]imidazole-2,4-diones which are useful as an intermediate for the preparation of biotin (vitamin H).
2. Description of the Related Art
Heretofore, as a method for producing hexahydrothieno [3,4-d]imidazole-2,4-diones using hexahydrofuro[3,4-d]imidazole-2,4-diones as a raw material, the following methods have been known.
1. A method in which hexahydrofuro[3,4-d]imidazole-2,4-dione is reacted with an alkali metal salt of thioacetic acid as disclosed in Helvetica Chimica Acta, 53, 991-999 (1970), PA1 2. a method in which hexahydrofuro[3,4-d]imidazole-2,4-dione is reacted with thioarnides as disclosed in Japanese examined patent publication Shou 62-7196, PA1 3. a method in which hexahydrofuro[3,4-d]imidazole-2,4-dione is reacted with an alkali metal salt of hydrogen sulfide as disclosed in Japanese examined patent publication Shou 51-17557, and PA1 4. a method in which hexahydrofuro[3,4-d]imidazole-2,4-dione is reacted with an alkali metal salt of O-alkylxanthic acid as disclosed in Japanese unexamined patent publication Hei 8-217779. PA1 a process for producing hexahydrothieno[3,4-d]imidazole-2,4-diones of the formula (1): ##STR2## wherein R is the same or different and represents a lower alkyl group, an alkenyl group, an aryl group or an aralkyl group all of which may be substituted, which comprises: PA1 reacting hexahydrofuro[3,4-d]imidazole-2,4-diones of the formula (2): ##STR3## wherein R is the same meaning as defined above with hydrogen sulfide in the presence of a basic compound and sulfur. PA1 an aprotic polar amide solvent such as N,N-dimethylformamide, N,N-dimethylacetamide, 1,3-dimethyl-2-imidazolidinone, tetramethylurea, hexamethylphosphoric triamide, N-methyl-2-pyrrolidone; PA1 an aprotic polar solvent such as dimethyl sulfoxide and sulfolane; PA1 a glycol including polyethylene glycol and a monoether and diether thereof such as ethylene glycol, 2-methoxyethanol, dimethoxyethane, diethylene glycol, diethylene glycol monomethyl ether, diethylene glycol dimethyl ether, triethylene glycol monomethyl ether, triethylene glycol dimethyl ether; PA1 a basic solvent such as N-methylmorpholine, diisopropylamine, triisopropylamine, tri-n-butylamine, .beta.-picoline, .gamma.-picoline, 2-methyl-5-ethylpyridine, quinoline, isoquinoline, N,N-dimethylaniline, N,N-diethylaniline, diazabicyclo[5,4,0]undec-7-ene; and a mixed solvent thereof.
However, the first method required stoichiometric amount of expensive alkali metal salt of thioacetic acid, and the second method also required expensive thioamide and was not satisfactory in yield. The third and the fourth methods are not always satisfactory in yield.